Embodiments are provided relating to noise filtering of seismic data, and in particular, but not by way of limitation, to filtering seismic data obtained from a sensor array in order to remove or attenuate ground roll.
In seismic surveys, a source induces seismic waves at or near the surface of the earth. These waves propagate through the earth and reflections from different layers within the earth can be detected by sensors, or geophones, at the earth's surface. The seismic source vibrations applied to the earth's surface also generate a so-called surface wave or ground roll which propagates through the shallow layers of the earth.
At the sensors/geophones, the time of incidence of the low frequency, low speed ground roll typically may coincide with the incidence of reflections from the deep layers of interest in the seismic survey. The simultaneous presence of the ground roll with the reflected signals may make it difficult to make full use of the seismic data as the ground roll often masks the reflected waves. Several methods are known for attenuating ground roll interference and thus reducing its effect on the seismic signal of interest. Typically, geophones are not used individually, but rather are connected in sub-arrays, or groups, which are hard-wired or summed together. This is a form of data-independent beam forming.
Adaptive signal processing has been applied in various forms in order to suppress the ground roll contribution.
U.S. Pat. No. 4,556,962 discloses a method for attenuating the ground roll from a surface seismic source by placing a sensor close to the source to detect the interfering noise. The interfering noise is scaled, delayed and summed with signals from a more distant geophone array and then cross-correlated with the original vibration source. U.S. Pat. No. 4,556,962 also suggests that an adaptive filter may be used so as to modify the delayed signal to correspond more closely to that detected by the more distant geophone array. However, the ground roll measured close to the source may be substantially different from that received by the geophone array, and the adaptive filter may not be able to account for this difference.
U.S. Pat. No. 4,890,264 discloses a method for suppressing non-uniformly distributed noise generated by surface wave propagation. In U.S. Pat. No. 4,890,264, horizontal geophones for detecting surface waves are used with conventional vertically orientated geophones for detecting seismic energy. The outputs of the surface wave detectors are used in conjunction with an adaptive filter to cancel the effects of the surface wave interference. This method for the suppression of ground roll is inherently a multi-component method. Some seismic wave energy also gets detected by the horizontally sensitive geophones, and this may cause signal cancellation.
GB-A-2273358 discloses linearly constrained adaptive beam forming and adaptive interference cancelling beam forming for ground roll suppression. This method filters signals measured by an array of geophones and sums them in such a way as to preserve signals incident from a preferred direction while suppressing interference incident from other directions. The filtering is performed using a continuously adaptive method with the moveout differential between the seismic reflections and the ground roll being used to form primary and reference channels. The suggested application for the disclosed method is in drilling when using a drill as a seismic source, where the ground roll is effectively stationary due to the slow travel of the drill bit and each source receiver position produces a lot of data. This ensures that the stochastic-gradient-type of algorithms used in the adaptive filters of this method are able to converge. However, in surface seismic experiments the ground roll present is often non-stationary and inhomogeneous and the stochastic-gradient-type of algorithms may be too slow to converge within the signal envelope.
U.S. Pat. No. 5,237,538 discloses a method for removing coherent noise from seismic data. This method firstly identifies the moveout characteristics of the noise, defines and extracts a space-time gate containing the noise, and removes the moveout to flatten the noise train. Amplitudes and time variations are then removed from the gate. The coherent noise is estimated using a beam steer operator (conventional stacking in this case) or by f-x filtering in the Fourier transform domain. The filtered noise estimate is subtracted from the data trace containing the signal-plus-noise using a short three to five point single filter. Inverse amplitude scalars are applied to undo the effect of earlier amplitude equalisation. The signal is then moveout restored into the original seismic record.
U.S. Pat. No. 5,971,095 disclosed a data adaptive principal component multi-channel filter bank to attenuate the coherent noise. This method works very well when the noise reference does not contain reflected signals. Like any other Least Square or Least Mean Square based adaptive noise canceller, this method assumes that the correlation of the signal and the noise is negligible compared with the correlation of the noise reference and the noise at the primary trace and its success depends on constructing the signal free noise references.